"Have been told that Medical Board's advice in late Sunanda's case has been received," Delhi Police Commissioner BS Bassi said in a tweet.

Bassi added that Special Commissioner of Police (Law and Order) Deepak Mishra was reviewing the progress. Sunanda was found dead inside her suite at a five-star hotel here in January 2014, a day after she was involved in a spat with Pakistani journalist Mehr Tarar on Twitter over the latter's alleged affair with Tharoor, the Congress MP from Thiruvananthapuram.

Her viscera samples were sent to the FBI lab in Washington DC in February last year to determine the kind of poison that killed her after an AIIMS medical board identified poisoning as the reason behind her death but did not mention any specific substance.

The FBI report had virtually rules out the theory of 'polonium poisoning' having caused her death. However, after Delhi police failed to draw any conclusion on basis of the findings, the report was handed over to a medical board for examination before further proceedings in the high-profile case.

The investigators have so far conducted polygraph test on six persons, all prime witnesses in the case, including Tharoors' domestic help Narayan Singh, driver Bajrangi and Sanjay Dewan, a close friend of the couple. Tharoor has been questioned in the case.